Generally, conventional power strips are susceptible to transient surge voltages resulting from lightning strike. For preventing the transient surge voltages from damaging the electrical appliances which are electrically connected to the power strip, the conventional power strip usually has a surge protective device for preventing damage of electrical appliances.
FIG. 1A is a schematic view illustrating a conventional power strip. As shown in FIG. 1A, the power strip 10 comprises a plug device 101, a conducting line 102 and a power strip main body 103. The power strip main body 103 includes a surge protective device 1031 and a plurality of electrical sockets 1032.
FIG. 1B is a schematic circuit diagram illustrating the surge protective device of the power strip shown in FIG. 1A. The surge protective device 1031 of FIG. 1B principles includes three metal oxide varistors (MOV) MOV1, MOV2, MOV3, and two thermal fuses F1, F2. When a surge is inputted into one of the Line terminal 1011, the Neutral terminal 1012 and the Ground line 1013, the surge will be transmitted to one of the other two terminals through the path R1, R2 or R3. Under this circumstance, the surge will not flow through the electrical sockets 1032 so as to protect the electrical appliances which are electrically connected to the electrical sockets 1032.
For example, if a surge is inputted into the power strip 10 through the Line terminal 1011, the surge will be successively transmitted through the thermal fuse F1 and the metal oxide varistor MOV1 and outputted from the Neutral terminal 1012. When the surge is received by the metal oxide varistor MOV1, the inherent high impedance of the metal oxide varistor MOV1 will become low impedance. Since a low impedance path R1 is defined, the surge will be transmitted to the Neutral terminal 1012 through the low impedance path R1.
Recently, as the requirements for electrical safety of electrical appliances become more stringent, stringent safety regulations associated with surge protective device are provided. Underwriters Laboratories Standard for Safety for Transient Voltage (UL 1449) is the primary safety standard for testing surge protective device. The second edition of UL 1449 proposes a lightning strike simulation test. In accordance with the lightning strike simulation test, when a voltage of 6,000 volts and a current of 500 amperes are inputted into an electrical appliance having a surge protective device, the surge protective device should exhibit a clamping voltage less than 330 volts.
On Sep. 29, 2006, UL 1449 has recently been revised, and will be introduced as a third edition. In accordance with the lightning strike simulation test proposed in the third edition of UL 1449, the surge protective device should also exhibit a clamping voltage less than 330 volts even if the test condition become more stringent (i.e. 6,000 volts, 3,000 amperes). Experiments demonstrate that most surge protective devices meeting the requirements of the second edition of UL 1449 exhibit a clamping voltage larger than 400 volts. In other words, the surge protective devices of the conventional power strips fail to meet the requirements of the third edition of UL 1449.
Therefore, there is a need of providing a power strip having a surge protective device to meet the requirements of the third edition of UL 1449 while having the advantages of simplified configuration, cost-effectiveness and industrial utility.